Caesium electric discharge device



June 10, 1952 A. W. HULL 2,600,246 CAESIUM ELECTRIC DISCHARGE DEVICE Filed April 19, 1951 a .9 & v /a 12 l n n 1:

Invent or: Albert W. Hull, hzru, 8%

His, Attorney.

Patented June 10, 1952 CAESIUM ELECTRIC DISCHARGE DEVICE Albert W. Hull, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 19, 1951, Serial No. 221,856

6 Claims. (01. 313-227) My invention relates to electric discharge devices of the type generally known as thyratrons which utilize an ionizable medium and a control grid, and, more particularly, to such devices employing caesium, rubidium or alloys thereof, as the ionizable medium.

The construction and use of a successful caesium vapor electric discharge device has long been a goal of the discharge device art, since it is known that caesium vapor possesses the lowest ionization potential, i. e., 3.9 volts, available and, therefore, that a caesium vapor device operates at a low arc drop resulting in very high efficiencies. Further, it has been found that in a caesium vapor device having a small reservoir of liquid caesium, no oxide coating or other special electron emitting surface must be applied to the cathode, since a monatomic layer of caesium is .condensed from the caesium vapor upon the hot cathode and serves as an efficient source of emitted electrons. Moreover, the emitting surface made up of a monatomic layer of condensed caesium has a very long life and high current capacity, since the monatomic layer of caesium is maintained on the cathode, even under severe operating conditions, by an equilibrium process of condensation on and evaporation of caesium from the cathode surface. Thus, a successful caesium vapor electric discharge device possesses the desirable characteristics of high efficiency, long life, high current capacity, small size, and ruggedness.

Howeventhe employment of caesium as an ionizable medium in an electric discharge device presents several serious obstacles which must be overcome before the device may be considered successful and practical. The problems relating to the construction of uncontrolled caesium discharge devices are specifically described in U. S. Patent 2,489,891, issued to me November 29, 1949, and assigned to the same assignee as that of the present invention.

In general, caesium is a chemically active element which attacks many materials, particularly glass insulators, employed in making a hermetically sealed envelope and insulating various electrodes from one another. To maintain sealed tightness of the envelope, caesium resistant ceramic, instead of glass, insulators and caesium resistant seals are used. However, caesium re-' sistant ceramics, while excellent insulators in air and at room temperature, behave quite differently in the presence of caesium vapor at elevated temperatures. It has been found that appreciable condensation of caesium on the surfaces of ceramic insulators considerably reduces their insulation properties producing a relatively low resistance surface leakage path and, therefore, that it is necessary to maintain the ceramic insulators at a higher temperature than the liquid reservoir of caesium in order that condensation does not occur in appreciable amounts on the insulators. The essential problem, then, is to maintain some portion of a caesium'discharge device at the proper temperature for the desired vapor pressure of caesium and to maintain the insulators, as well as electrodes, at higher temperatures in order that caesium is not condensed in appreciable amounts thereon. The present invention is concerned with additional problems encountered in the construction of a grid controlled caesium discharge device-and constitutes over the said patent an improvement which provides a successful and practical grid controlled caesium vapor discharge device.

In the construction of a successful grid controlled caesium vapor discharge device even greater problems are encountered, since two ceramic insulators are required between the three electrodes and since the control grid itself must be maintained above the lowest temperature in the device. The temperature of the control grid, as well as that of the anode, must be prevented from reaching the emission temperature of a caesium-coated surface or grid emission and anode emission will occur. Grid emission results in loss of grid control, while anode emission may initiate arc-back. Consequently, for almost every part of the device, the temperature must be neither too high nor too low, being thus restricted to a relatively narrow temperature range. I i

It is an object of my invention to provide a new and improved grid controlled electric discharge device of the type employing caesium as an ionizable medium.

It is a further object of my invention to provide a necessary temperature controlling system for a grid controlled caesium vapor electric discharge device.

It is a still further object of my invention to provide a grid controlled electric discharge de vice of the caesium type requiring less critical temperature control through the use of a caesium-alkali metal alloy.

In carrying out my invention in one form thereof, I provide an electric discharge device having a cylindrical shell, with suitable end closures, forming the greater part of ahermetically sealed envelope and serving also as an second ceramic insulator bonded between the. control grid structure and the. anode. shell and,

forming a part of the sealed envelope. An outer jacket and headers are provided. around; the. sealed envelope to define a pathforcooling fluid;

between the anode and the jacket; andthe control grid is made up of tubular members. sup.- ported between headers to define a path for cooling fluid through the control grid. amount of caesium is present in the sealed envelope, lying; in. liquid form. in a small trough near the botton'iv end of..the.- anode. shell, or in some. other portion so located with respect to the circulation path of the: cooling fluid that its temperature is lower than that of any other part of the enclosure.

In operation, the tubeis heated, sufileiently, by cathode heating means or external means, so that aportion of the caesium vaporizes to the proper; vapor pressure. The. cathode is also heated. and. emits electrons. which; arecollected by theianode. held at. positive potential with re= specii to. the: cathode. Inorder-that the various parts of thedevice. are.- keptat. the proper tezn= pcratures. in. accordance: with the aforemem tinned. requirements; cooling; fluid is circulated serially through the outer. jacket: passing by, the first ceramic, insulaton, the; caesium reservoir, the: anode shell;.and; the second; ceramicinsulator in that order; and. thence through the. tubular control grid; The liquid caesium. is thus kept cooler than any part in the tube, sincethe fiuid passes nearit first". The; first insulator is; kept warmer than the liquid caesium, even. though the: fluid passes linearly in the flow cycle, since iii is. positioned to. receive.- radiated heat from thelheated cathode stem. The cooling. fluid cools the. anode and absorbs heatfrom. the, anode so that by the time the. fluid passes thesecondinsulator it; is warmer thantheliquid caesiumand thus keeps the second insulator higher in temperature than the liquid caesium. The gridis nextcooled by fluid flow but only to a. tempera.- ture. somewhat above the temperature of the liquid caesium. Therefore; by this arrangement, any appreciable condensation of caesium vapor occurs only at the coolest point, i. e., back into the: liquid caesium, and not on the anode, control grid, or insulators. An appreciable con.- densation is one considerably inexcessof a monatomic layer producing a lowered. surface leakage: resistance.

The; novel features of my inventionare pointed out with particularity in. the appended claims. However, for a better understanding of themventiom. together with. furtherobjects and. advantages thereof, reference should be hadjt'o the following" description, taken. in conjunction with the: accompanying drawings, wherein:-

Fig. l is an elevational view: in'section of an electric discharge device illustrating one embodimentof: my invention; and Fig. 2 is a crosssectional view takenalong the line.2--2 of Fig; 1.

Referring now to Figs- 1 and 2,. I have. shown an electric discharge. device comprising a her- A. small.

metically sealed envelope including a cylindrical shell I, the inner surface of which serves, in this preferred form, as the anode of the device. To close the lower end of shell i, an annular header 2 is provided through which a cathode metal support cylinder 3 extends. In order to support cylinder 3 and the cathode structure from header 2:, and. insulate the cathode; from electrical connection with the-anode; means including an insulating seal are provided, in this instance a cylindrical insulator 4 made of a material not attacked. by caesium, such as ceramics of the alumina or of the magnesium silicate groups. As shnwn. thedraiving, a metal collar '5 is bonded at one end thereof to support cylinder 3 and at the other end thereof to a metal connecting sleeve 6 which-,in. turn, is joined to insulator A preferable method of efiecting a reliable seal between metal sleeve 6 and ceramic insulator 6 is one such as the titanium hydride method disclosed. and. claimed: in the copending application of. Bondley, Serial No. 36,244, filed January 30, 1948, and. assigned. to they same assignee as. that of the presentapplication. A second metalconnectingsleeve '1- is. bonded,. in a, manner-similar to that for sleeve 6, to. the. outer surface of insulator 4, well spaced from the; inner.- end of insulator 4, and welded. to the. header 2, asshown, to complete the lower; portion of the sealed envelope.

The. cathode: structure includes a. plurality of vanes 8: which extend; radially. from. cylinder 3 and. which may be covered: with screen. mesh. .9 to. increase. the. effective areazand; emission ca.- pacity of. the. cathode. Cylinder 3; vanesfi, and mesh 9- are. preferably? made oii'clean nickel, no special emitting: surface: being required. in. this caesium. vapor device. as explained hereinbeiore. A. heat conservingshieldlil is supported. from cylinder 3 to.surround vanes. 8, beingas showrra generally closed. cylindrical metallic covering having a' plurality of; layers? H." of thin. metal: or metal foil: and: provided with openings 12: in the side: walls through which electrical. discharge may take; place between the.:anode;and; the' oathode; The interior." ofsupport cylinder: 3. serves as an. hermetically sealed. chamber containing a heating element [3,. preferably" tungsten. Heat'- ing element i3 is wound upon a ceramic-sleeve i l which-surrounds the. inner end; ofia metallic support and electrical leadi-inv member [5,. to. which one end of element I3 is welded. The other: end of. element. i3; is. welded to astud 16 extending inwardly-from a cap. or closure. member [T which closes the innermost; end of the chamber. Support and lead-in member [5 extendsthrough. an insulating; seal to the; exterior of; the chamber, such a seal. being formed by a connectingcylinder I8 Weldedat one end therof: to: the innerisurface. of cylinder 3 and sealed; at: the outer end to a; glass cylinder (.9. A closing: collar 20' is sealed to cylinder I9 and, in turn, welded. to member l5. Member ['51 maybe spaced: from cylinder 3 by a heataresistant' insulation. ring 21:, as'shown, if it2istnecessary or: desirable. Heating current is supplied to member I5 anda. ter.:-- minal 22 which, being attached to cylinder 3, serves as the cathode terminal;

Onthe inner surface of header Zthere is located: a. stiffening'ring 23' having an inwardly'exitending flangeld'. Ring Zdserves to strengthen the relatively-thin header 2' in supporting the cathode structure, while flange 24 provides an annular reservoir between itself and shell I to hold a small amount of liquid metal 25, which may be caesium, rubidium, or certain alloys thereof giving an ionizable medium having the desired features of caesium described herein before. v

The upper end of shell I is closed by a second annular header 26, also having a stiffening ring 2'1, through which a metal support cylinder 28 extends to support the control grid structure. The sealed envelope is completed by a collar 23 bonded at one end thereof to cylinder 28 and at the other end thereof to a metal connecting sleeve 30. Sleeve 30, in turn, is sealed to a cylindrical insulator 3|, similar to insulator 4, by a process such as the process given by the aforementioned copending application. As shown in the drawing, another metal connecting sleeve 32 is also sealed to insulator 3| and also bonded to header 26 to complete the upper portion of the sealed envelope.

In order to provide for cooling of the control grid, a unique structure, preferably made of iron, is supported from cylinder 28, and comprising a pair of ducts 33 and 34 welded into cylinder 28 and communicating with a ring header 35 into which the upper ends of a plurality of grid-forming tubes 36 are fastened. Supported within cylinder 28 by an annular disk 31 is a tube 38, the inner end of which communicates with the interior of the sealed envelope and on the upper end of which there is provided a seal-01f tubulation 39 for reducing the pressure in the envelope and making the final seal. Annular disk 31 also closes the lower end of the annular space between cylinder 28 and tube 38 which is a part of the cooling fluid flow path, as will be explained hereinafter. As shown more clearly by Fig. 2, the annular space between cylinder 28 and tube 35 is divided by a diametral partition ll] and ring header 35 is divided by a similar diametral partition 4|, providing a cooling passageway, in order that cooling fluid may flow downwardly in half of grid-forming tubes 36 and upwardly in the other half of tubes 36, the bottom of ends of tubes 36 being fastened into another ring header 62, as shown in Fig. 1. end of cylinder 28 is a fitting 53 to conduct the flow of cooling fluid, as will be explained hereinafter, and around fitting 43 a control grid terminal 44 may be conveniently located. For controlgrid cooling purposes, a small opening 35 is made through cylinder 26, as shown.

To provide an outer jacket for the device and define a path for cooling fluid which gives the proper temperatures at the various parts, I have welded an outer cylinder 46 around anode shell I and separated them with a spiral bafiie 4?, as shown in Fig. 1. Headers 2 and 26 are also enclosed by the jacket, since closure members 48 and 49 are bonded to the inner surface ,of shell I at its opposite ends. To the inner edges of closure members 48 and 56, bellows 50 and 5I are bonded, as shown, in order that expansion or undue pressure will not cause a leak in the jacket and, at the same time, providing a path for fluid flow past insulators 4 and 3I. The jacket is completed by sleeves 52 and 53, bonded to bellows 56 and 5!, and sealed into glass insulators 54 and 55; and by other sleeves 56 and 51 sealed into insulators 56 and 55 and bonded to collars 53 and 59, which, in turn, are bonded to cylinders 3 and 23. An entrance 6!] to the cooling jacket is formed on sleeve 56, the ultimate exit being formed by fitting 43. In order to define spiral paths of fluid flow past insulators and 3! and to thus obtain good heat exchange relation therewith, bafiles 6| Attached to the upper and 62 are mounted respectively on sleeves I and 32; and similarly spiral fluid flow paths past headers 2 and 26 are defined by bafiies 63 and 64, mounted on closure members 48 and 49.

Since the device is intended to operate with the coolest portion of the tube, i. e., the caesium reservoir defined by flange 24, at around C. so that the caesium vapor pressure is approximately 10 microns, it is preferable to provide an external sheathed heating element 65 around cylinder 46 to bring the device up to temperature before commencing operation and to hold it there during standby periods. Of course, the cathode heating element I 3 may be used for this purpose, but its action is slower and, therefore, less con venient. Once the device is in operation,'the heat generated therein is more than suflicient to maintain the desired temperatures and the cooling jacket and circulating cooling fluid acts to maintain each part of the device at the proper temperature. It will be apparent that the cooling fluid to be used must be one which is electri cally non-conductive and which is unaffected by the relatively high temperatures involved. Water, of course, is not satisfactory but certain oils such as the silicone oils have been employed with success as cooling fluids for the device illustrated.

In operation, current is conducted by electric discharge from anode to cathode through an anode terminal 66 and cathode terminal 22. However, as in well known thyratron type discharge devices, 'such discharge can only be initiated when the control grid is at or above a predetermined potential, applied to terminal 44, for the particular voltage difference between the anode and the cathode. Electron emission, as previously mentioned, occurs from a monatomic equilibrium layer of caesium on the hot cathode, heated by heating element I3 to approximately 750 C. Electrons striking the control grid and the anode produce a certain amount of heat which must be removed in order to maintain the various parts of the device at the proper temperatures mentioned hereinbefore, and this is accomplished by the device illustrated in Fig. 1 by cooling fluid circulated in the following path: Relatively cool fluid enters the device through entrance 66, passes insulator 4, goes through baffles EI and 63, and thence through an opening 61 in shell I and into the space between shell I and cylinder 46. The liquid caesium 25 held by shell I, header 2, and flange 24 lies near the early part of the cooling fluid flow cycle and is, therefore, maintained as the coolest part of the device. Even though insulator G is surrounded by cooling fluid in an earlier part of the flow cycle, its inner surface is maintained at a temperature from 10 C. to 50 C. above the temperature of the liquid caesium by heat radiated directly from cylinder 3 which is relatively hot due to heat re ceived from element I 3. After entering the space between shell I and cylinder 46, the cooling fluid flows in the spiral path around shell I, cooling the anode and being warmed as a result. The fluid next flows through a second opening 68 in shell I, past header 26 in a spiral path defined by baffle 64 and upwardly past insulator 3I in a spiral path defined by bafile 62. The fluid, after passing anode shell I, is warmer than when it passed liquid caesium 25 and, therefore, insulator 4| is maintained above the lowest temperature in the device and there is little likelihood of caesium vapor condensing on the surface of insulator 3|. Also, it will be noted that the inner surfaces, i. e.,

accuses through opening 45:, thence. passing downwardly through. the: left: half. of; the; annular space, into. ductl33, through; the-.lefthali. of ring header, 35 and: downwardly: through: the-left, half of grid tubes 3%. Entering: ring header t2, the cooling fiuhll fioivs then 1. wardly through theri htthalf. of grid; tubes: 33; through duct upwardly be tween: cylinder; 23; and tube; 33,1, andv out. of. the; cooling: jacket through. fitting: 43, The: grid structure-is; therefore,- also cooled by the circulatiorrof cooling fiuidandithe dangerofitbecomingan-emitter, asthecathode is, isremote. However, the grid structure; by-thismethod; is. maintained. appreciably, hotter: than the coolest portionin the device; andthe danger of caesium conidcnsing'on the; grid, structure in appreciable amounts; is :also remote.

The=caesiumdischarge deviceshown by Figs. 1 and 2 is, therefore, one which successfully'meets the:problems:encounteredin the construction of a; controlled caesium vapor device, The; various partspff the device; are marintainedatrthe proper temperatures: for successful. operation by the unique:- sequentia l series flow path for" coolin fluid.

ihe ioni-zable medium: employedin the. device of? my invention. be; caesium or; rubidium. however, I, have. discovered, that: an, alloy of ca-csiumionrubidium .with'somej other: alkali metal such": as potassium. or. sodium. may be used, to further advantage, and; therefore, I prefer in some, cases to" use a caesium-sodium alloy, for example. for the ionizable medium. The, hensfitzoi: anpallov such. as' this is believedto, result from a" lower: caesium vapor pressure over: the liquid alloy-than over: pure. caesium, with the con.- sequence; that caesium vapor tends to condense more easily back. into the liquid pool and less easilyon to the ceramic insulators, anode, ore-ontrol grid. Therefore, the temperature difierentials:betweenthe-variousparts of the device, are

Vapor pres. of Us abovc;al1oy Vapor; pres. of'Gs above Cs t/Oll'llg peI' cent of'Cs in alloy From; the above expression it will be seen that a id-5.0 atomic percent alloy of" caesium with another alkali metal, say potassium, Will result in onl'yone-half the caesium vapor pressure over the liquid metal for a given temperature than that resulting from the use of pure caesium, Hence, the use of'such an alloy gives the-caesium vapor more tendency to condense back into the li'quidmetal instead of onto the insulators and electrodes, with the result that the temperature difierential between the liquid metal and the various other parts of the tube is not such a critical factor to the successful operation of a dis charge device of this type. In certain cases, however, it is desirable to use pure caesium or:

8; rubidium ,asthe ionizable-zmedium. vwhile: 111 other cases; alloys; of these;- metals; with, other." alkali; metalsimay: bra-used .wlthadvantage.

While the present invention has been describfifii by. reference to. particular. embodimentszthereof, itpwill be understoodjthat numerousmodifications maybemadevby thoscskilled inthe artiwithout actuallyrdepartingifrom the invention. I, therefore, aim in theappended claimsto, coverallsuch equivaientvariations as; come, withinthe; true; spiritrandscope; of tho-foregoing disclosure.

What. I, claim. as new and. desire; to secure: by. Letters-Patenuof; the United States: is:

L. Av grid: controlled; electric; discharge. device: comprisingan hermeticallysealcdenvelopawith: an anodetherein; air-quantity of-;liquid.metal set lected', from the, group consisting of; caesium. rubidium, alkali metal alloys; of caesium, and alkali metal alloys of rubidium in aportion: of said envelope; a; cathode structurermeans supporting said cathode structure within said,en-.- velope in insulated relationwith respect thereto including 2. first ceramic insulator formingpart of the envelope Wall'and positioned to receive heat from saidcathode-structure during the operation of said device;- a control grid structure; means supporting said control grid structurewithin said envelope in insulated relation with respect there-- toincluding a second ceramic insulator forming part of the envelope-Wall; and means definingcooling paths'in heat exchange relation with said portion of said envelope, one electrode of said device, and said second insulator in series so that during operation of said device-cooling fluid may be circulated through said paths in the order named to maintain said. second insulator at a temperature above the temperature of said liquid metal.

2. An electric discharge device of the thyratrontype" comprising an hermetically sealed envelope; a quantity of liquid metal selected from the g'roup consisting of caesium, rubidium, alkali metal alloys of' caesium, and alkali metal alloys of rubidium Within said envelope; a cathode structure; means supporting said cathode structure within said'envelope in insulated relation'thereto including a first ceramic insulator forming part of the envelope wall; a control grid structure ineluding a pair of headers and a plurality of tubular members connected between said headers defining a cooling path therethrough; means supporting said control grid structure within said envelope in insulated relation with respect thereto including a second ceramic insulator," said first ceramic insulator being positioned to receive heat from said cathode structure; and means defining a cooling fluid flow patharound said, second insulatorin series with said control grid cooling path.

3. A grid controlled electric discharge device comprising: an hermetically sealed envelope in cluding a metal portion forming an anode for said device; a quantity of liquid metal selected from the-group consisting of caesium, rubidium, alkali metal alloys of caesium, and alkali metal alloys ofrubidiumwithin said envclopera cathode structure; means supporting said cathode structure within said envelope in insulated relation with respect thereto including a: first ceramic insulator forming part of the envelope Wall and positioned to receive heat from said cathode structure durmg operation of said device; a control grid structure; means supporting said control grid structurewithin said envelope in spaced relation between said cathode structure andsaid envelopeand in insulated relation with respect to said envelope including a second ceramic insulator form ing part of the envelope wall; means defining cooling paths in heat exchange relation with the region containing said liquid metal, said anode portion of said envelope, said second insulator, and said control grid structure so that during operation of said device cooling fluid may be circulated serially through said paths in the order named to maintain said liquid metal at a first predetermined temperature, to maintain said anode portion and said control grid structure below a second predetermined temperature but above said first predetermined temperature, and to maintain said second insulator above said first predetermined temperature.

4. An electric discharge device of the thyratron type comprising an hermetically sealed metal envelope forming an anode for said device, a reservoir defined in one end of said envelope, a quantity of liquid caesium in said reservoir, a cathode structure, a first ceramic insulator supporting said cathode structure from said one end of said envelope and positioned to receive heat from said cathode structure during operation of said device, a control grid structure including a plurality of tubular members supported in spaced relation between headers defining a cooling path therethrough, a second ceramic insulator supporting said control grid structure from the opposite end of said envelope, and jacket means around said envelope and said second insulator defining cooling paths in heat exchange relation therewith,

10 said jacket means and control grid structure being arranged so that cooling fluid may be circu- 'lated past said envelope and said second from said one end of said envelope and positioned to receive heat from said cathode structure during the operation of said device, a control grid structure, a second ceramic insulator supporting said control grid structure from the opposite end of said envelope, and jacket means definin a cooling path around said anode and said second insulator in the order named so that during the operation of said device cooling fluid may be circulated through said jacket means to maintain said liquid caesium at a first predetermined temperature, to maintain said anode below a second predetermined temperature but above said first predetermined temperature, and to maintain said second insulator above said first predetermined temperature.

ALBERT W. HULL.

No references cited. 

1. A GRID CONTROLLED ELECTRIC DISCHARGE DEVICE COMPRISING AN HERMETICALLY SEALED ENVELOPE WITH AN ANODE THEREIN; A QUANTITY OF LIQUID METAL SELECTED FROM THE GROUP CONSISTING OF CAESIUM, RUBIDIUM, ALKALI ALLOYS OF RUBIDIUM IN A PORTION OF ALKALI METAL ALLOYS OF RUBIDIUM IN A PORTION OF SAID ENVELOPE; A CATHODE STRUCTURE; MEANS SUPPORTING SAID CATHODE STRUCTURE WITHIN SAID ENVELOPE IN INSULATED RELATION WITH RESPECT THERETO INCLUDING A FIRST CERAMIC INSULATOR FORMING PART OF THE ENVELOPE WALL AND POSITIONED TO RECEIVE HEAT FROM SAID CATHODE STRUCTURE DURING THE OPERATION OF SAID DEVICE; A CONTROL GRID STRUCTURE; MEANS SUPPORTING SAID CONTROL GRID STRUCTURE; MEANS ENVELOPE IN INSULATED RELATION WITH RESPECT THERETO INCLUDING A SECOND CERAMIC INSULATOR FORMING PART OF THE ENVELOPE WALL; AND MEANS DEFINING COOLING PATHS IN HEAT EXCHANGE RELATION WITH SAID PORTION OF SAID ENVELOPE, ONE ELECTRODE OF SAID DEVICE, AND SAID SECOND INSULATOR IN SERIES SO THAT DURING OPERATION OF SAID DEVICE COOLING FLUID MAY BE CIRCULATED THROUGH SAID PATHS IN THE ORDER NAMED TO MAINTAIN SAID SECOND INSULATOR AT A TEMPERATURE ABOVE THE TEMPERATURE OF SAID LIQUID METAL. 